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With an in-cylinder gas sampling system that was modified from a gasoline direct injection injector, a series of in-cylinder gas sampling experiments were performed in a Ricardo Hydra four-stroke single-cylinder gasoline engine to study the components and concentrations of chemical substances in the hot residual gas by using methanol and gasoline, respectively. And then the effects on low-temperature oxidation process of these chemical substances were analyzed with the experimental data and chemical reaction mechanisms. The experimental results show that there were some chemical substances, such as formaldehyde, acetaldehyde in the trapped hot residual gas achieved by negative valve overlap method when fueled with methanol or gasoline. These components activated the chemical reactions in the cylinder and promoted the low-temperature oxidation of the fuel. The concentration of formaldehyde in the hot residual gas of methanol was significantly higher than that under the same experimental conditions of gasoline because of the easier production process of formaldehyde in methanol according to reaction mechanisms. And the concentration of formaldehyde decreased in the prophase of the compression process and then increased rapidly, which indicated that formaldehyde in the mixture accelerated the oxidation process by participating in the reaction and that is one of the reasons why temperature increased rapidly and ignition timing advanced in the methanol-controlled auto-ignition combustion process. With the increasing residual gas fraction, formaldehyde concentration in the mixture decreases for lower reactant concentration and this affected the ignition timing as well as the heat release rate accompanied by the temperature. [ABSTRACT FROM AUTHOR]